Conventional agar-based assay systems have been adapted (a) to evaluate agents exhibiting differential in vitro test results in the Stage I cancer screen, (b) to identify and quantitate cell populations which may survive such treatment, and (c) to compare profiles of cell line drug sensitivity observed in Stage I and Stage II assays. Also, the development of an under-agar microculture tetrazolium assay system appears well-suited for accurate drug sensitivity determinations of compounds which liberate cells from fluid monolayer culture systems (i.e., false-positive test results due to chemical and/or physical properties of the test material). Such a culture format is amenable not only to conventional colorimetric analysis but also to image analysis which affords a far greater detection range (4-5 logs) and which does not require the use of organic solvents. In addition, we are developing methods to support new in vivo/in vitro drug assay strategies based upon the culture of pre-formed colonies and xenograft-cultivated material (coupled with ex vivo bioassays) which should help provide experimental insights concerning in vivo drug efficacy (or lack thereof). Overall, the agar-based assay protocols are designed (a) to identify agents which appear to have the greatest potential for in vivo efficacy and (b) to eliminate agents lacking appropriate pharmacologic profiles, and (c) to permit isolation and subsequent characterization of drug-sensitive and -insensitive cell populations within screening panel cell lines.